In a turbocharged engine system, intake air is compressed using torque derived from expanding engine exhaust. The compressed—i.e., boosted—intake air increases the power output of the engine relative to that of a naturally aspirated engine of the same displacement. Accordingly, addition of a turbocharger may allow a vehicle to be driven by a ‘downsized’ engine, for better fuel economy, while still maintaining acceptable power for vehicle launch.
However, a conventional turbocharger may exhibit an acceleration lag when the exhaust-driven turbine is spinning too slowly to provide enough torque to the intake-air compressor. When tip-in is initiated from this state, exhaust temperature and pressure may build slowly, because the engine—possibly a downsized engine—is operating with little or no boost. This problem may become more noticeable at higher altitudes, where the barometric pressure and oxygen content of the ambient air is reduced. It may be especially noticeable in automatic-transmission vehicles, where drive shaft and axle are coupled through a torque converter. A conventional torque converter may exhibit a so-called stall speed for the drive shaft, below which the torque converter will not develop sufficient torque to move the vehicle. As a result, the net acceleration lag may be exacerbated such that it takes several seconds to accelerate the vehicle, even on a level surface. Naturally, if the vehicle is oriented uphill, the lag may be greater still, resulting in driver dissatisfaction.
Various approaches have been advanced to address the issue of turbocharger lag. For example, U.S. Patent Application Publication Number 2010/0155157 describes a turbocharger operable as an electrically driven supercharger under conditions of inadequate exhaust energy. Solutions like this, which require extensive additional hardware, may be costly to implement. Other approaches address limited aspects of turbocharger lag via control-system modifications. For example, U.S. Pat. No. 6,488,005 describes a diesel engine in which idle speed is increased if the vehicle is oriented uphill, thereby providing a greater reserve of compressor torque. However, this approach, among others, fails to contemplate the range of remedies that may be taken to reduce turbocharger lag, as well as the range of observables useful for predicting when such remedies may be applied.
Therefore, one embodiment provides a method of operating a turbocharged engine of a motor vehicle, the engine having a cylinder which undergoes a compression stroke. The method includes, for a given engine speed and load, advancing an ignition spark in the cylinder by a first amount relative to top dead center (TDC) of the compression stroke at a first barometric pressure. The method also includes advancing the ignition spark by a second, lesser amount relative to TDC of the compression stroke at a second, lower barometric pressure, for the same engine speed and load. Operating according to this method, a suitably configured engine will release higher-temperature exhaust at the lower barometric pressure, providing more energy to spin up the turbine, for faster accumulation of boost.
The summary above is provided to introduce a selected part of this disclosure in simplified form, not to identify key or essential features. The claimed subject matter, defined by the claims, is limited neither to the content of this summary nor to implementations that address problems or disadvantages noted herein.